The present invention relates to a device and a method for monitoring a pressure sensor, which is positioned in a braking system of a vehicle.
Conventional devices and method for monitoring a pressure sensor are available. For example, the following procedure for monitoring a pressure sensor is described in the article xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d [ESPxe2x80x94electronic stability program of Bosch] published in the automotive engineering journal ATZ Automobiltechnische Zeitschrift, vol. 96, issue no. 11 (1994) pp. 674 through 689: In order to monitor a pressure sensor, in this case an admission pressure sensor, a so-called active pressure sensor test is carried out. In this active pressure sensor test, braking pressure is fed into the braking system, using a pump included in the braking system, preferably using a precharge pump. At the same time, it is checked whether the pressure sensor signal is acting plausibly.
This kind of monitoring has the disadvantage that an active pressure buildup is required to carry out the pressure sensor test, and this is realized with the aid of a precharge pump. As a result, this pressure sensor test cannot be used for braking systems which do not have a precharge pump at their disposal. This kind of pressure sensor test also has the disadvantage that, during malfunctioning of the precharge pump, monitoring the pressure sensor is no longer possible.
Consequently, an object of the present invention is to create a device and a method for monitoring a pressure sensor which, respectively, do not require an active pressure buildup. That is, monitoring a pressure sensor is to be realized, without the use of a precharge pump.
German Patent Application DE 197 55 112 A1 does not describe a method or monitoring device for carrying out a pressure sensor test. However, this application describes a method and a monitoring device for determining fading of the braking action of a motor vehicle brake. To do this, the longitudinal acceleration of the vehicle is measured. The measured longitudinal acceleration is related to an ascertained braking operation. From this is determined whether the measured vehicle longitudinal acceleration corresponds to the braking operation. To determine the braking operation, in particular the braking pressure in a brake fluid line or the excursion of the brake pedal are measured.
According to an example embodiment of the present invention, a device for monitoring a pressure sensor positioned in a braking system of a vehicle includes means whereby a first vehicle movement variable is ascertained during a braking action, which characterizes a vehicle motion in the longitudinal direction. The device, according to the example embodiment further includes means with which a mass variable is ascertained that characterizes the mass of the vehicle. Advantageously, for monitoring the pressure sensor, the device according to the present invention includes means with which are ascertained whether the first vehicle motion variable lies within a value range for the first vehicle motion variable, whose limits are ascertained as a function of the mass variable.
Since the first vehicle motion variable is a function of the vehicle mass, this procedure ensures that, for monitoring the pressure sensor, the first vehicle motion variable is compared to a xe2x80x9cmatchingxe2x80x9d value range, that is, to a value range adapted to the mass of the vehicle.
Advantageously, the first vehicle motion variable is ascertained during a braking action performed by the driver. To do this, only such braking actions are registered in which a regulating device contained in the vehicle does not execute any interventions on the vehicle brakes independently of the driver. Advantageously, the mass variable is ascertained during at least one drive operation. To do this, only such drive operations are registered in which a regulating device contained in the vehicle does not execute any interventions on the propulsion independently of the driver. Because, as the braking actions and/or the drive operations, only those are registered, on which no driver-independent interventions in the vehicle brakes and/or the propulsion are being carried out, one makes sure that monitoring of the pressure sensor in so-called stable operating states of the vehicle is being carried out. Due to this procedure for monitoring the pressure sensor, no precharge pump is necessary, since, at least within the framework of this monitoring, no driver-independent braking actions are necessary or rather, are taken into account. Thus, the monitoring of the pressure sensor according to the example embodiment of the present invention can also be used for braking systems which have no precharge pump, but instead have a pneumatic booster. Additionally, the pressure sensor can also be monitored if a precharge pump malfunctions in a braking system equipped with a precharge pump.
Using the pressure sensor monitored by the device and the method according to the present invention, an admission pressure variable is ascertained which describes the admission pressure set by the driver. As part of the sensor system, the device according to the present invention may be further provided with means with which wheel speed variables are ascertained, which describe the wheel speeds of the individual wheels. Advantageously, the first vehicle motion variable is ascertained as a function of the admission pressure variable and the wheel speed variables.
Advantageously, in the device according to the present invention, means are provided with which a first vehicle deceleration variable is ascertained, at least as a function of the admission pressure variable. This represents the vehicle deceleration to be expected theoretically on account of the operation of the brake by the driver. Ascertainment of the first vehicle deceleration variable is advantageously made by using a mathematical model. As parameters, this contains nominal values, i.e., fixed, predefined variables for the braking torque translation, namely, the wheel radius as well as the vehicle mass. Advantageously, the first vehicle deceleration variable is ascertained in a small timing window. Thus, one may assume that interfering influences, such as wind forces, downgrade forces or driving resistance forces have no influence.
In the device according to the present invention, further means are advantageously provided with which a second vehicle deceleration variable is ascertained, as a function of the wheel speed variables of the rear wheels. This represents the vehicle deceleration which actually occurs during the braking action. In order that the second vehicle deceleration variable represents the actually occurring vehicle deceleration as accurately as possible, those wheel speed variables of the wheels are used for ascertaining it which are less subject to slipping during a braking action. During a braking action, these are the rear wheels.
For the purpose of ascertaining the first vehicle motion variable, the first and the second vehicle deceleration variable are brought in relation to each other. In this connection, use is made of the fact that, during partial braking, a correlation exists between the admission pressure, which enters into the first vehicle deceleration variable and the vehicle deceleration that occurs, which is represented by the second vehicle deceleration variable. In the case in point, for the ascertainment of the first vehicle motion variable for successive time steps, in each case a relationship is formed between the present first and second vehicle deceleration variable, respectively. The first vehicle motion variable is formed as the average of these relationships. Since there is a connection between the first vehicle motion variable and the sensitivity of the pressure sensor, this first vehicle motion variable can be used to monitor the pressure sensor, in particular, to monitor the sensitivity of the pressure sensor.
As described above, the mathematical model for ascertaining the first vehicle deceleration variable includes as a parameter a fixed, predefined value for the vehicle mass. Since, however, depending on the loading of the vehicle, the vehicle mass can differ, this should be taken into account in monitoring the pressure sensor. Therefore, the limits of the value range, mentioned above, are ascertained as a function of the mass variable. Thereby, tighter error recognition thresholds can be used for monitoring the pressure sensor. The mass variable is advantageously ascertained as a function of a torque variable which describes the drive torque delivered to each drive wheel.
Corresponding to the ascertainment of the first vehicle motion variable, the mass variable is ascertained as a function of a first and a second vehicle acceleration variable. For this purpose, in the device according to the present invention, means are provided with which the first vehicle acceleration variable can be ascertained, at least-as a function of the torque variable. This describes the vehicle acceleration to be theoretically expected during the drive procedure. The first vehicle acceleration variable is advantageously ascertained with the aid of a mathematical model, in which the wheel radius as well as the vehicle mass are used as parameters. Corresponding to the first vehicle deceleration variable, the first vehicle acceleration variable is also ascertained in a small time window. This ensures that interfering proportionate shares in the acceleration, which can come about, for example, through wind forces, downgrade forces or driving resistance forces, have no influence on the ascertainment of the first vehicle acceleration variable, because, during this small time window, they may be assumed to be constant, and thus eliminated.
Furthermore, in the device according to the present invention, means are provided with which the second vehicle acceleration variable is ascertained as a function of the vehicle speeds of the wheels not driven. This describes the vehicle acceleration which actually occurs. Since the second vehicle acceleration variable should describe the actually occurring acceleration as accurately as possible, in correspondence to the procedural manner in the braking action, in the drive procedure as well, those wheels are taken into consideration, which are less subject to slipping. In the drive procedure, these are the wheels not driven.
In the device according to the example embodiment of the present invention, means are advantageously provided, with the aid of which, for successive time steps, in each case a relationship is formed of the respectively present first and second vehicle acceleration variable. From these relationships, a second vehicle motion variable is formed as an average value. By forming an average value, the information quality of the second vehicle motion variable is increased, because not only a single point in time, but various points in time, and thus various first and second vehicle acceleration variables enter into the ascertainment of the second vehicle motion variable. The mass variable is ascertained as a function of the second vehicle motion variable. With reference to the information quality of the first vehicle motion variable, in relation to the first and the second deceleration variable, the same thing applies correspondingly.
Advantageously, for the ascertainment of the mass variable, the second vehicle motion variable is compared to the predefined comparison values for the second vehicle motion variable, and the mass variable is ascertained as a function of these comparisons. In this connection, it has proven sufficient, using the mass variable, to distinguish at least between a lightly loaded, a heavily loaded and an overloaded vehicle. In the case of a lightly loaded vehicle, first limits are selected for the value range, and second limits are selected in the case of a heavily loaded vehicle. In this connection, the spread of the first limits is less than the spread of the second limits. The limits ascertained as a function of the mass variable determine the value range of the first vehicle motion variable. Since the first vehicle deceleration variable enters into the ascertainment of the first vehicle motion variable, which is ascertained with the aid of a mathematical model which has nominal values of the vehicle mass and the braking torque transmission, and since the actually present variables of the vehicle mass and the braking torque transmission can be different from the nominal values, the value range has to be adapted to the actually present vehicle states at least as a function of the vehicle mass which, as described above, may be estimated, in order that one can perform meaningful monitoring of the pressure sensor. In this connection, in the case of greater vehicle mass, the value range must be selected larger on account of greater possibilities of fluctuations, so that a more certain monitoring of the pressure sensor can be undertaken. In the case of an overloaded vehicle, no monitoring of the pressure sensor is performed, since in this case, the fluctuation of the vehicle motion variable is too great, and thus, a meaningful monitoring of the pressure sensor is no longer possible.
Several drive procedures, successive in time, are advantageously evaluated for ascertaining the mass variable. Correspondingly, to determine whether the pressure sensor is defective or not, several braking actions, successive in time, are evaluated. By evaluating several procedures, the quality of each variable or the information is improved. Because, before the value of the mass variable is fixed or before a statement is made about whether the pressure sensor is faulty or not, the observation results which enter into the measured variable or the information have to be reproducible several times, one after the other. That means, that a faulty value or a faulty information on account of an outlier in the observation results is excluded by this procedure.
During a starting period of the vehicle engine, which is determined by the driver-dependent starting and stopping of the vehicle engine, advantageously, at least one monitoring of the pressure sensor is carried out. This means that, during the time span defined by turning the ignition key to start the vehicle engine and by pulling the ignition key to stop the engine, at least one monitoring of the pressure sensor is carried out. In the following discussion, this starting period is denoted as an ignition cycle.
Advantageously, the status of the pressure sensor present during the preceding starting period is taken into account in monitoring the pressure sensor during the current starting period or the current ignition cycle. The process of the monitoring of the pressure sensor during the current starting period is determined as a function of this status. Thus, in the case that, during the preceding starting period, no malfunction of the pressure sensor was determined, monitoring the pressure sensor is carried out with the aid of the value range, whose limits are ascertained as a function of the mass variable. On the other hand, in the case that, during the preceding starting period, a malfunction of the pressure sensor was determined, a modified monitoring of the pressure sensor is carried out with the aid of a value range whose limits are predefined in a fixed manner. In this connection, the spread of these limits predefined in a fixed manner is greater than the spread of the limits ascertained as a function of the mass variable.
As mentioned above, at least one monitoring of the pressure sensor is carried out during one starting period. In this connection, an attempt is made to begin the monitoring of the pressure sensor as rapidly as possibly after starting the vehicle engine, and to terminate it as soon as possible, i.e., to be able to make a statement as soon as possible as to whether the pressure sensor is faulty or not. However, in the framework of monitoring the pressure sensor, as was mentioned above, since several braking actions and several drive procedures are being evaluated, the time span required for monitoring the pressure sensor has different values, depending on the manner of drive of the driver.
In the above description, the concept of the preceding starting period is used, which says implicitly that, starting from the current starting period, clearly, the directly preceding starting period is to be taken into accounted. In this connection, one of the previous starting periods can also be taken into account, or several of the previous starting periods.
If no fault of the pressure sensor is determined in the modified monitoring of the pressure sensor, advantageously, subsequently to the modified monitoring of the pressure sensor, a monitoring of the pressure sensor is carried out with the aid of the value range, whose limits are ascertained as a function of the mass variable. This means that the regular, i.e., customary monitoring of the pressure sensor follows the modified monitoring of the pressure sensor.
It has proven of particular advantage, to terminate monitoring of the pressure sensor in the case where a fault of the pressure sensor is determined in response to the modified monitoring. Since both during the preceding starting period the pressure sensor was recognized as being faulty, and in response to the modified monitoring of the pressure sensor a fault was again determined, the pressure sensor is assumed to be constantly faulty. That is why renewed monitoring of the pressure sensor is no longer required.
Advantageously, during the performance of the modified monitoring, a regulating device included in the vehicle, for which a pressure variable, ascertained with the aid of the pressure sensor, represents an initial value, is switched to passive.
It is of particular advantage, for the case in which the monitoring carried out with the aid of the value range whose limits are ascertained as a function of the mass variable, a fault of the pressure sensor is determined, if a regulating device included in the vehicle, for which a pressure variable, ascertained with the aid of the pressure sensor, represents an initial value, is switched off. This manner of proceeding secures that no faulty interventions in the actuator technology takes place. Alternatively, instead of switching off the regulating device, an operation under emergency conditions is provided to set in, in which it has reduced regulating functions.
During the monitoring of the pressure sensor, its sensitivity is advantageously checked.